The Role of E3 Ligase HRD1 in Bile Acid Synthesis and Primary Sclerosing Cholangitis

A. PROJECT SUMMARY Primary sclerosing cholangitis (PSC) is a chronic liver disease, histologically characterized by a progressive course of cholestasis with inflammation and fibrosis of the bile ducts, and clinically leading to multi-focal biliary strictures and with time cirrhosis and liver failure1, 2. Synthesis of bile acids (BAs) plays an important role in liver injury in cholestasis, and the process is regulated by cholesterol 7α-hydroxylase (CYP7A1) and 3 beta-hydroxysteroid dehydrogenase type 7 (3β-HSD7, HSD3B7) which are both exclusively located in the hepatic endoplasmic reticulum (ER)3-6. Here, we identify a novel feedback regulatory mechanism in bile acid synthesis, the “bile acid-HRD1-HSD3B7 axis”, complements the canonical bile acid synthesis regulatory mechanism (Figure 1): (1) E3 ubiquitin ligase HRD1 is induced by bile acids in the liver; (2) Bile acids induced HRD1 suppresses bile acid synthesis through interacting with and mediating ubiquitination and degradation of the HSD3B7, the key enzyme in bile acid synthesis; (3) HRD1 liver-specific knockout (HRD1Alb) mice display increased bile acid synthesis and develop chronic liver injury; (4) and unlike the classic regulatory mechanisms of bile acid synthesis, the newly-identified bile acids-HRD1 feedback regulatory axis modulate the bile acid homeostasis at the post-translational level by directly targeting HSD3B7 for protein degradation. In this application, we will test that bile acids induces E3 ligase HRD1 elevation to increase ubiquitination of HSD3B7, which induces HSD3B7 protein degradation by the 26S proteasome and consequently inhibits bile acid synthesis and cholestatic liver injury as a new feedback regulatory mechanism. By modulating both ER stress induced liver injury and bile acid synthesis, E3 ligase HRD1 is an ideal target for PSC treatment.